In a fuel injecting device of the internal combustion engine, it is necessary to supply the fuel depending on the driving conditions of the engine. Particularly, the air fuel ratio should be restricted within a narrow window area around a stoichio by this device in order to highly and effectively employ a three way catalyst converter for purifying the exhaust gas. It is also necessary to maintain the air fuel ratio at a certain objective value around the stoichio.
On the other hand, an air fuel ratio required for the internal combustion engine differs depending on its load and engine speed, and, for example, as shown in FIG. 10, it is preferable to set the objective air fuel ratio in accordance with the load in the areas, such as a fuel cut area, a lean area, the stoichio area, and a power area. Particularly, in order to accommodate low fuel consumption, a lean burn engine has been developed which can be generally driven within the lean area.
An internal combustion engine carries out feedback control that detects sensed air fuel ratio signals over a wide range by an air fuel ratio sensor, calculates a set air fuel ratio by which the difference can be eliminated between the sensed air fuel ratio and an objective air fuel ratio determined depending on the driving conditions, and actuates a fuel injection valve in order to secure a fuel injection amount corresponding to the set air fuel ratio, thereby adjusting the air fuel ratio at the objective air fuel ratio over a wide range.
For driving the internal combustion engine in a manner described above, it is very important to precisely control the air fuel ratio into the objective value with respect to improvement of the fuel consumption, improvement of the engine power, stabilization of the idling rotation, improvement of the exhaust gas, and improvement of drivability. Thus, it is desired to improve reliability and stability of detected values of a large area air fuel ratio sensor.
Now, problems to be solved by the present invention are as follows:
That is, to judge a jam or a trouble is important for improving the reliability and the stability of the large area air fuel ratio sensor (LAFS). Generally, an output of the sensor may be varied from around 0 (v) to a sensor supply voltage Vs, and may be kept at an intermediate voltage on jamming. Thus, it is difficult to diagnose a sensor jamming merely on the basis of an output range on judging the jam of the large area air fuel ratio sensor.
Accordingly, it has been proposed to calculate the set air fuel ratio in order to eliminate a deviation between the objective air fuel ratio and the sensed air fuel ratio, thereby carrying out jam judgment for the large area air fuel ratio sensor under the set driving condition of the engine in accordance with the sensed air fuel ratio, the set air fuel ratio, and the deviation therebetween.
However, such a conventional method yields a lag between an air fuel ratio setting time and an air fuel ratio measuring time due to, for example, a transporting process of the fuel injected in an intake path of the engine, a process lag and a detection lag of the sensor. Thus, when the sensor output is simply compared with the sensed air fuel ratio in such manner, there is a defect that the sensor jam judgment will be roughly made in spite of the engine being driven in a constant condition, and it is impossible to correctly judge the sensor jam.
Accordingly, a primary object of the present invention is to provide an air fuel ratio control device for an internal combustion engine which accurately judges a jam of the large area air fuel ratio sensor to improve the reliability of the sensor detected value as well as to provide an air fuel ratio control device for an internal combustion engine which enables the air fuel ratio control to be carried out precisely.